A known process in mechanics is the cold permanent deformation of one end of a fastener, such as a nail, a rivet, a pin, a stud, an eyelet and so on, in order to form a head (or a counter-head, should the member be already provided with a head at the opposite end) at that end so as to obtain a fixed and non-removable connection of the pieces, e.g. section bars, sheets, rolled sections and so on, through which the fastener is arranged.
Said process, which can be either a manual or a mechanized process, is generally referred to as “setting” and, in case of mechanized processes, the tools employed will be referred to as “riveting machines”.
In bodywork construction, especially but not only in aeronautical field, use of so-called shear rivets is known for fixedly connecting sheets. An example of such rivets is disclosed in GB 2 420 835.
FIGS. 1a and 1b show a rivet of the kind mentioned above applied to a pair of sheets L1, L2, before and after riveting, respectively.
This kind of rivet is generally denoted 11 and it includes a stem or shaft 13 having a head 15 and a threaded or grooved end 17a, and a collar 19 which is set on a threaded or grooved portion 17b of stem 13, by applying a relative axial traction between end 17a of stem 13 and collar 19. Said axial force causes moreover breakage of end 17a at a fracture zone, denoted by reference numeral 21, generally coincident with a corresponding annular groove.
Riveting machines designed for setting rivets of the above kind are also known. An example of such machines is disclosed in U.S. Pat. No. 6,766,575.
According to the prior art, the riveting process comprises the following steps:                in a first step, rivet 11 is inserted into a hole previously formed through the pieces to be connected;        collar 19 is then fitted onto rivet 11;        rivet 11 is then inserted into the head of a riveting machine, which holds rivet stem 13 inside a chuck equipped with jaws;        the front part of the machine head, generally consisting of a moving plate, is moved away from the head body, thereby causing the setting of collar 19 and the consequent breakage of end 17a of the stem;        the front part of the machine head is then moved back against the body and the machine is ready for receiving a new rivet that, when entering the head, will cause ejection, through a rear opening provided therein, of end 17a or tail that has remained seized between the jaws.        
One of the problems arising when designing a riveting machine for rivets of the above kind is related with the need of getting rid of the rivet end or tail, once the latter has been broken during the setting operation, so that the machine is ready for performing a new working cycle.
In currently employed machines, the tail is generally disposed of by exploiting the push of the tail of the subsequent rivet, which is introduced into the machine head thereby ejecting the tail of the previous rivet that has remained seized between the head jaws. Yet, this method is not suitable for use in all applications.
For instance, according to the prior art, the tail can be ejected only through the rear opening in the riveting machine head. Moreover, since ejection takes place by means of a push, jamming can frequently occur due for instance to blocking of the tail or the subsequent rivet, thereby causing machine stop.
More particularly, the known method is not suitable for applications in which the channel housing the rivet tail inside the riveting machine head is used also for the passage of a flow of cooling air.
Such an air flow is generally employed for cooling the sheet surfaces during drilling performed prior to rivet insertion.
Once drilling is completed, the rivet is inserted into the hole formed in the sheets, possibly upon application of a sealing material, and is then fixed by setting.
If, after the breakage of the tail, the latter remains seized inside the machine head until the arrival of the subsequent tail, the channel for the passage of the cooling air flow would be obstructed and the riveting machine head could not be used for cooling the surfaces when drilling the subsequent hole.
A further problem arising when using the prior art riveting machines is the impossibility of freeing the rivet from the head jaws once the rivet has been seized in order to perform the setting step.
Actually, it is clear that, if for any reason, for instance a fault in rivet manufacture or a wrong positioning of the riveting machine head, the operator realizes that the riveting operation cannot be properly performed, or that the tail did not break because of the above problems, an intervention on the riveting machine head will be necessary, in order to try to unlock the jaws for releasing the rivet.
This problem is particularly serious in case of automated, robot-controlled riveting machines. In such case indeed it would be necessary to manually operate in order to release the rivet from the head, often by disassembling part of the head, in positions which can hardly be reached by the hands of an operator and with working delays, incompatible with and automated or robotized process.
Thus, it is an object of the present invention to solve the above problems by providing a head for a riveting machine that allows a self-release of the rivet.
It is a second object of the invention to provide a head for a riveting machine that allows ejecting the rivet tail.
It is another object of the invention to provide a head for a riveting machine that allows clearing the channel inside the machine head after setting, thereby making said channel available for the passage of a cooling air flow.
It is a further object of the invention to provide a head for a riveting machine that, besides solving the above problem, can be constructed by simple and cheap modifications of the existing heads.
It is yet a further object of the invention to provide a method of operating a head for a riveting machine, which method allows solving the problems mentioned above and can be applied in existing heads, with a limited number of changes